This invention relates to a catalytic process for preparing alkylene glycols, preferably ethylene glycol, from alkylene oxide and water.
Alkylene glycols, such as ethylene glycol and propylene glycol, are widely used as raw materials in the production of polyesters, polyethers, antifreeze, solution surfactants, and as solvents and base materials in the production of polyethylene terephthalates (e.g. for fibers or bottles). Commercial processes for the preparation of alkylene glycols typically involve the liquid phase hydration of the corresponding epoxide in the presence of a large molar excess of water (see, e.g., Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 11, Third Edition, page 929 (1980)). The primary by-products of the hydrolysis reaction are di-, tri-, and higher glycols. However, as compared to monoalkylene glycols, the demand for di-, tri-, tetra-, and polyalkylene glycols is low. The formation of the di- and polyglycols is believed to be primarily due to the reaction of the epoxide with alkylene glycols. As epoxides are generally more reactive with glycols than they are with water, a large excess of water is employed in order to favor the reaction with water and thereby obtain a commercially attractive selectivity to the monoglycol product. However, even in light of the large excess of water, a typical commercially practiced method for making ethylene glycol has a molar selectivity to monoethylene glycol (MEG) of between 80 to about 90 percent (%), a molar selectivity to diethylene glycol (DEG) of between 9 to about 15%, and a molar selectivity to triethylene glycol (TEG) of between 1 to 5%. In addition, increasing the water to epoxide feed ratio also increases the cost of distilling water from the glycol. Thus, there is much interest in alternative processes that increase monoalkylene glycol selectivity without increasing production costs.
One such alternative is a heterogeneous catalytic process such as the use of a selectivity-enhancing metalate anion-containing material. See, for example, EP-A-156,449. Typical metalate anions consist of anions of molybdate, tungstate, metavanadate, hydrogenpyrovanadate and pyrovanadate. Such a process can demonstrate acceptable conversions, good selectivity, and a low water/alkylene oxide ratio. Moreover, U.S. Pat. Nos. 4,277,632 and 4,982,021 disclose the use of a pH adjusting additive to enhance the performance of the metalate-containing materials. However, a disadvantage of such processes is that the alkylene glycol-containing product stream also comprises a substantial amount of metalate anions which have been displaced from the electropositive complexing sites of the solid metalate anion-containing material. Therefore, an additional separation step is required in order to remove the metalate anions from the product.
One variation of a heterogeneous catalytic process is based on catalytic hydration of ethylene oxides in the presence of carbon dioxide and an anion-exchange resin in the halogen form. See, for example, JP-A-57-139026. Such halogen type anion exchange resins include halides of chlorine, bromine, and iodine, and in particular, basic anion exchange resins. Disclosed as being especially suitable is a chloride form anion exchange resin such as DOWEX.TM. MSA-1, which is an anon-exchange resin containing benzyl trimethyl ammonium groups as electropositive centers. A disadvantage of this process is that the product stream contains a mixture of both glycols and carbonates. Isolation of the glycols from the mixture is difficult because the boiling temperatures of di-glycols and carbonates are close to each other. In addition, separation of ethylene glycol is further complicated due to the close relative volatility of ethylene glycol and ethylene carbonate at low concentrations of ethylene carbonate in ethylene glycol.
Yet another variation utilizes a similar process of reacting alkylene oxide and water in the presence of carbon dioxide, but utilizes a bicarbonate form of the anion exchange resin. See, for example, Russian Patent Nos. 2002726 and 2001901. In the cited Russian publications it is specifically disclosed to use Anionites AV-17 and AV-17-T as the anion exchange resins. These are disclosed as polystyrenes cross-linked with divinylbenzene and having quaternary ammonium groups in the bicarbonate form. The Russian publications further disclose use of carbon dioxide in amounts ranging from at least as low as 0.01 weight percent (wt %). This variation of the process attempts to eliminate the difficult separation of the alkylene glycol product from the carbonate, but it still suffers from the disadvantage of having an undesirably low productivity or activity at temperatures that do not cause rapid loss of catalyst activity (&lt;130.degree. C.). The activity can be improved by operating at higher temperatures, but the catalyst rapidly loses activity at high temperature, thus the catalyst must be replaced often. Large amounts of catalyst are required in either case. Furthermore, the selectivity is relatively low compared to systems that operate without carbon dioxide.
In WO/20559A, it is pointed out that the aforementioned Russian publications (similar to the halogenate-type resin publications) do not dispense with the addition of carbon dioxide to the feed. According to WO/20559A, carbon dioxide is detrimental to the catalytic effect of bicarbonate-exchanged resins of the quaternary ammonium type and it is disclosed to perform the process in the substantial absence of carbon dioxide. However, the process described in WO/20559A suffers from the disadvantage of having an undesirably short catalyst lifetime and undesirable resin swelling at reasonable temperatures (e.g. &gt;95.degree. C.).
One alternative for improving the desirability of the aforementioned types of anion exchange resins is to improve the activity of the catalyst by increasing the temperature at which the process is conducted. However, one potential drawback of the aforementioned conventional types of anionic exchange resins is their limited tolerance to high temperatures. Therefore, one publication discloses a catalyst system which employs a polymeric organosiloxane ammonium salt (see WO 97/19043) and another publication discloses a catalyst system which employs a bicarbonate form of an ion exchange resin that contains, as electropositive centers, nitrogen atoms linked to two or more atoms other than methyl group carbon atoms (see WO 97/33850). Both of these publications disclose their catalyst systems as being solutions to the potential problem with more conventional anion exchange resins which have been found under severe reaction conditions (high temperature and/or long service) to have unacceptable deterioration of alkylene glycol selectivity. One disadvantage of such catalyst systems is that they are typically more expensive as compared to the more conventional systems such as MSA-1 type catalyst.
It is desirable to have longer catalyst lifetimes and higher activity for the alkylene glycol preparation process, while utilizing the more conventional anion exchange resins.